The Treaty of Hamburg, signed on 5 March 1638, was a diplomatic agreement between France and Sweden that formally ratified the earlier Treaty of Wismar, which had been negotiated in March 1636. The treaty is considered a significant turning point in the Thirty Years' War (1618–1648).
Background
Following the Swedish and Protestant defeat at the Battle of Nördlingen in 1634, France recognized the need for greater involvement to prevent Holy Roman Emperor Ferdinand II from winning the war. The Treaty of Compiègne (April 1635) had been an initial attempt to support the Swedes, but the Protestant position deteriorated further when Saxony switched sides under the Peace of Prague (May 1635) and the Protestant Heilbronn League dissolved. In May 1635, France also entered open war with the Spanish Habsburgs. By October 1635, the Protestant general Bernard of Saxe-Weimar and his formerly Swedish mercenary army were taken into French service, leaving cash-strapped Sweden to hold northern Germany with only a handful of northern allies.
The Swedish victory at the Battle of Wittstock in Brandenburg (October 1636) demonstrated that Sweden remained a capable military power. However, before the Treaty of Hamburg was signed, Swedish Field Marshal Johan Banér was driven from Saxony to Pomerania by Imperial forces under Matthias Gallas in 1637.
Terms
Under the terms of the treaty, France agreed to pay Sweden 1,000,000 livres for its military contributions against the Habsburgs. The accord also confirmed the Franco-Swedish alliance as established by the Treaty of Wismar, which had been negotiated in March 1636. The alliance prohibited either party from negotiating a separate peace with the Habsburgs.
Significance
The treaty provided Sweden with crucial financial resources to continue its military campaigns, enabling the Swedes to go on the offensive again. It solidified the Franco-Swedish alliance that would persist through the remainder of the Thirty Years' War, ultimately contributing to the Peace of Westphalia in 1648.
